List patterns: counterexample for non-exhaustive switch expressions #55327

alrz · 2021-08-01T16:37:46Z

Relates to test plan #51289

Co-authored-by: Julien Couvreur <jcouv@users.noreply.github.com>

src/Compilers/CSharp/Portable/Binder/PatternExplainer.cs

jcouv · 2021-10-20T21:45:41Z

src/Compilers/CSharp/Portable/Binder/PatternExplainer.cs

+                    int lengthValue = lengthValues.Sample.Int32Value;
+                    if (slice != null && lengthValues.All(BinaryOperatorKind.Equal, lengthValue))
+                    {
+                        // Bail if there's a slice but only one length value is remained.


I wonder if we can support some of these cases, as this could be useful. Consider: null or { Length: 4 } => 0, [_, .. var middle, _] => 0 (that'd be a good test to add).
Drawing the distinction between cases we can correctly explain and those we can't may be tricky.
In draft #57210 I'd probably gone a bit too far in trying to explain more slice scenarios, but it may give you some idea.

We can also punt for now and file a follow-up issue to refine.

I added a test but it didn't hit this check. I think it's actually unlikely to get here without a nested length test.

It may be good to say why we bail (both cases).

Sorry, I see there was a typo in my example. Should have said not 4

Then it would be exhaustive. (no error)

jcouv · 2021-10-20T21:48:39Z

src/Compilers/CSharp/Portable/Binder/PatternExplainer.cs

+                            case BoundDagIndexerEvaluation e:
+                                var indexerTemp = new BoundDagTemp(e.Syntax, e.IndexerType, e);
+                                int index = e.Index;
+                                if (index > lengthValue - 1)


Do we need this check? #Closed

It would hit if we didn't check slice/lengthValues above. But I think it's safer to just check all this upfront to avoid a crash. The explainer is generally defensive on out-of-bound errors and such. See tryHandleTuplePattern for instance.
~~Should check the other end as well.~~ (added)

jcouv · 2021-10-20T21:49:45Z

src/Compilers/CSharp/Portable/Binder/PatternExplainer.cs

+                                continue;
+                            // A list pattern can only support a single slice within.
+                            // We won't try to generate a list pattern if there's more.
+                            case BoundDagSliceEvaluation e when slice == null:


nit: We can remove the when clause and use an assertion instead (Debug.Assert(slice is null);) #Closed

See comment above.

Would it help to add a Assert.Fail in code paths that we don't have a test scenario for?

~~I'd even throw UnreachableException, since we won't get here with multiple slices.~~
Never mind

jcouv

Done with review pass (iteration 8)

…the slice.

jcouv · 2021-10-22T16:54:46Z

src/Compilers/CSharp/Portable/Binder/PatternExplainer.cs

+                        if (slice.StartIndex - slice.EndIndex > lengthValue)
+                        {
+                            // Bail if the sample value is less than the required minimum length by the slice.
+                            return null;


~~Is this reachable? If not, consider throwing UnreachableException instead.~~
Ah, saw your comment about the explainer being defensive. #Closed

jcouv · 2021-10-22T16:58:08Z

src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs

+        var src = @"
+_ = new C() switch
+{
+    null or { Length: 4 } => 0,


Should this be not 4?

jcouv

LGTM Thanks (iteration 12) modulo one test nit

jcouv · 2021-10-22T17:00:03Z

@333fred @dotnet/roslyn-compiler for second review. Thanks

333fred · 2021-10-22T18:51:28Z

src/Compilers/CSharp/Portable/Binder/PatternExplainer.cs

+                    return null;
+
+                if (!constraints.All(c => c switch
+                {


Can we indent this? The current indentation level hurts my head.

333fred · 2021-10-22T19:13:27Z

src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs

        };

-        _ = this switch // didn't test for not null first element
+        _ = this switch // we didn't test for [.. null] but we're looking for an example with Length=1. incorrect explanation [.. null, null] // 1


incorrect explanation [.. null, null]

Let's remove this bit.

333fred · 2021-10-22T19:14:04Z

src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs

+                //         _ = this switch // we didn't test for [.. null] but we're looking for an example with Length=1. incorrect explanation [.. null, null] // 1
+                Diagnostic(ErrorCode.WRN_SwitchExpressionNotExhaustiveForNull, "switch").WithArguments("_").WithLocation(20, 18),
+                // (27,18): warning CS8509: The switch expression does not handle all possible values of its input type (it is not exhaustive). For example, the pattern '_' is not covered.
+                //         _ = this switch // didn't test for [.. [not null]] // // 2


It general, it would be good to add an explanatory comment in this file somewhere that these are the ideal explanations, but we're not able to handle these cases.

333fred · 2021-10-22T19:19:58Z

Done review pass (commit 13). Just a bit of formatting cleanup.

333fred

LGTM (commit 14)

dotnet-issue-labeler bot added the Area-Compilers label Aug 1, 2021

alrz changed the title ~~List patterns: generate counterexample for non-exhaustive switch expressions~~ List patterns: counterexample for non-exhaustive switch expressions Aug 2, 2021

WIP

cc00129

alrz force-pushed the list-patterns-explainer branch from bcdc483 to cc00129 Compare October 18, 2021 19:42

alrz mentioned this pull request Oct 18, 2021

Explainer for list-patterns #57210

Closed

alrz and others added 7 commits October 19, 2021 00:53

WIP

e9eb399

Cherry-pick tests from dotnet#57210

7dc6ac4

Co-authored-by: Julien Couvreur <jcouv@users.noreply.github.com>

Update baseline

4b29226

Skip discarded slice pattern

abd7676

Bail if there's a slice but only one length value is remained

3012db0

Reorder

b5b117e

Remove comments

2bf9fc4

alrz marked this pull request as ready for review October 20, 2021 19:24

alrz requested a review from a team as a code owner October 20, 2021 19:24

jcouv self-assigned this Oct 20, 2021

jcouv added the New Feature - List Patterns label Oct 20, 2021

jcouv reviewed Oct 20, 2021

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src/Compilers/CSharp/Portable/Binder/PatternExplainer.cs Outdated Show resolved Hide resolved

jcouv reviewed Oct 20, 2021

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alrz added 4 commits October 22, 2021 18:59

Add test

ddbc0c6

Bound check

7121a44

Reword

7dc82d4

Bail if the sample value is less than the required minimum length by …

b3db969

…the slice.

alrz force-pushed the list-patterns-explainer branch from d8bccf6 to b3db969 Compare October 22, 2021 16:16

alrz requested a review from jcouv October 22, 2021 16:17

jcouv reviewed Oct 22, 2021

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jcouv added this to the 17.1.P1 milestone Oct 22, 2021

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runfoapp bot mentioned this pull request Oct 22, 2021

Light bulb menu showed up over Pick Members dialog #57321

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jcouv mentioned this pull request Oct 22, 2021

Test Plan for "List Patterns" #51289

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jcouv modified the milestones: 17.1.P1, 17.0, 17.1 Oct 27, 2021

jcouv merged commit 94ea14c into dotnet:features/list-patterns Oct 27, 2021

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List patterns: counterexample for non-exhaustive switch expressions #55327

List patterns: counterexample for non-exhaustive switch expressions #55327

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List patterns: counterexample for non-exhaustive switch expressions #55327

List patterns: counterexample for non-exhaustive switch expressions #55327

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